1. Field of the Invention
The invention is in the field of coupler systems for railway cars, and in particular, the invention is directed to a novel knuckle, adapted to engage with American Association of Railroads (“AAR”) type E and type F couplers.
2. Description of Related Art
In a railway car coupler system, the knuckle element is the final point of contact between two railway cars, and the most prone to failure. In some sense, the knuckle is designed to fail; because if any element in the coupler system is to fail, it should be the lightest and most accessible element, which is the knuckle. However, given the constraints of size, shape and weight, it is still desirable that a knuckle should be made as resistant to fatigue and failure as possible. Knuckle failure accounts for an estimated 11,000 train separations a year, or about 30 separations per day.
Conventionally, a knuckle weighs approximately 78 to 88 pounds. However, a solid metal object this size, would weigh much more than the standard weight—upwards of 100 lbs. Therefore, conventionally, knuckles have been designed to contain internal voids to reduce the total weight, using a “core” in the casting process to create the internal void. For example, U.S. Pat. No. 7,337,826 B2 discloses and claims a method of using a core to obtain a cast knuckle having an internal void. One evident problem with the core method is that it produces inconsistent results. The core cannot be seen during the casting process, and it can move, causing the position of the internal voids and the internal wall thickness vary significantly in the finished products, with the result that the average fatigue life of knuckles is not consistent. This has led to a drive by the AAR to create fatigue life standards, as described below.
It has been discovered that relatively small point to point contact surfaces of the engaged portions of knuckles in a coupler system can cause premature failure due to stress risers being established within the knuckle. The inventor herein has recognized, based on industry studies and research, that these failures originate in the casting process, for example where the mold shifts along the parting line and a detrimental point to point surface contact is established in the finished knuckle. Grinding and/or machining of such imperfect surface after heat treatment can add substantially increased costs and creates crack initiation sites on the surface, thereby adding stress to the coupler knuckle and potentially resulting in premature and unpredictable knuckle failure.
Another reason for knuckle failure is the draft angles which are generally required in order to produce a satisfactory sand casting. Typically, a mold cavity is made using a pattern which has slight draft angles, often in a range of about 2 degrees to 3 degrees, in order to allow the pattern to be withdrawn from the mold cavity. Without the draft angles, withdrawing the pattern from the mold cavity can result in the sidewalls partially collapsing or otherwise deforming. However the draft angle yields a non-uniform contact surface area on the pulling face of the finished product. The present invention discloses an improved knuckle with improved contact surface engagement with other knuckles by virtue of having no draft angle. The invention herein provides a cast knuckle made without internal voids and without using cores, which still achieves the standard weight and has improved average fatigue life. A casting according to the present invention has no parting line caused by the meeting point of two mold halves.
Another inherent problem with the sand casing process is the porosity caused by moisture. In the sand casting process, moisture is required to hold the sand together. When the molten metal is poured into the mold, the moisture flashes to steam and may produce internal and surface porosity in the finished product.